Testing gravity’s Lorentz invariance

Today’s best fundamental theories—whether for gravity, electrodynamics, or elementary particles—say that the laws of physics are identical for all inertial observers, independent of their speed or direction of motion. That so-called local Lorentz invariance has been well tested for quantum field theories (see Physics Today, July 2004, page 40 ). To date, however, the LLI of gravitational interactions has received little attention, mostly because the weakness of gravity requires exquisitely sensitive experiments. In general, LLI tests are examined within the “standard model extension,” which incorporates a series of coefficients, nine of which reflect gravitational effects. Any nonzero coefficients demonstrate violations of LLI and could reveal clues about quantum gravity, variants on general relativity, or physics beyond the standard model. Some previously undetermined coefficients have now been pinned down by Holger Müller of the University of California, Berkeley, and his colleagues. Using an atom interferometer with an atomic fountain, they looked for anomalous variations in the gravitational acceleration g as Earth revolves through space. The physicists combined new results with those from previous experimental runs and with lunar-ranging data (see Physics Today, May 1996, page 26 ). The bottom line? Of the nine independent gravitational coefficients, five are now known to be zero to within parts per billion, and three to parts per million. One remains undetermined. The team also established that further improvements can come from using horizontal devices—perhaps guided atoms. (K.-Y. Chung et al., Phys. Rev. D 80 , 016002, 2009 http://dx.doi.org/10.1103/PhysRevD.80.016002 .)